The invention relates to improvements in a structure and method for quickly and simply measuring the load in a nip between two opposed rolls at different locations along the nip with different overall loads on the rolls. More particularly, the invention embodies positioning a thin bar in the nip between rolls with the bar providing an electrical read-out selectively indicating the load in the nip at different locations along the nip so that the operational factors in the nip can be quickly and easily tested.
While the invention is acceptable to uses in various types of machines, it is particularly well adapted to use in a paper making machine for measuring the nip load between rolls such as are used in the press section or calender or other locations in the machine, and the invention will be primarily described as used in that environment.
In a paper making machine, the pressures along the nip are very important because they will determine the characteristic of the paper web being produced from the machine. In the press section, for example, the pressure along the nip will determine the amount of water extracted from the traveling web and nonuniformity of pressures can result in nonuniformity of extraction and a consequent nonuniform web. Where this situation occurs, correction factors have to be provided further down the machine to compensate for this nonuniformity. Also, in calendering uniformity along the length of the nip is important if the web is to be uniformly calendered. It is also important to detect the pressures along the nip because in certain circumstances, controlled variable pressures are needed to provide correctional treatment to a web which has been produced nonuniformly in the fourdrinier section and has a nonuniform basis weight or moisture content. This situation is true for paper machines of all widths, although with very wide machines having widths in excess of 30 feet, the situation becomes more important. The practices for testing nip loads have followed various procedures, and a common method has been to place carbon paper in the nip at different locations, but this provides a relatively crude reading of nip pressure.
In many of the nips in paper making machines, the rolls are covered with composition materials such as those formed of an epoxy resin with a polyester reinforcing. Covering materials are sold under the tradenames of Ventex and Beltex, and these materials are very hard in some circumstances. With a very hard nip, the nip width becomes more narrow, and the distribution of pressure along the nip must be kept very even for satisfactory results. A hard nip is very sensitive to any error in the crown match between the rolls.
Accordingly, it is an object of the present invention to provide an improved mechanism and method for testing the load in the nip between opposing rolls at selective locations along the nip and at different overall nip loads more easily, simply and accurately than heretofore possible.
Another object of the invention is to provide a nip load sensing device which can be placed in a nip prior to operation and will provide a visual read-out of the load in the nip at different locations along the nip length.
A feature of the invention is the provision of a thin elongate bar preferably formed of a material the same as the cover of a roll forming one of the pressure rolls, with the bar having a plurality of sensing load cells cast therein at frequent spaced locations axially along the bar so as to be centered in the nip load with the load cells testing the stress on the bar and providing an electrical output indicative of the load at that location.
Other advantages and features will become more apparent as will equivalent structures which are intended to be covered hereby, in the teachings of the principles of the invention in connection with the disclosure of the preferred embodiment thereof in the specification, claims and drawings, in which: